The present invention relates to a servo parameter detection method for manufacture of optical pickup devices to be mounted on information recording/reproduction apparatuses, as well as to an optical pickup device using the servo parameter detection method.
In order to detect an optimum value of a servo parameter required for recording and reproduction on optical discs such as BDs (Blu-ray Discs), DVDs (Digital Versatile Discs) and CDs (Compact Discs), there arises a need for optimum parameter detection using a reproduction signal or its jitter.
In recent years, there has been a demand for higher density of optical discs. For achievement of this higher density, advancements have been made toward enhancement of track recording density or narrower pitches of tracks in the information recording layer of optical discs. It is also needed for the achievement of higher density of optical discs to reduce the beam diameter of an optical beam converged on the information recording layer of an optical disc. Means for achieving the reduction of the beam diameter of the optical beam could be to increase the numerical aperture (NA) of the optical beam outputted from an objective lens as a converging optical system of an optical pickup device, and to shorten the wavelength of the optical beam.
When an optical beam from a light source passes through a cover glass of an optical disc, there arises a spherical aberration. Since the magnitude of the spherical aberration is, in general, proportional to the fourth power of a numerical aperture, use of an objective lens having a high numerical aperture would involve a matter of errors of the spherical aberration, which would have influences on information reading. Therefore, use of an objective lens having a high numerical aperture needs to be accompanied by high-accuracy correction of the spherical aberration.
It is also necessary that the focus servo meets high tracking performance so that the beam spot diameter keeps generally constant at all times. Generally, the focus servo needs to be controlled within a range enough smaller than the depth of focus that is proportional to the wavelength of a light source and inversely proportional to the square of the numerical aperture of the objective lens. Further, when the optical pickup inclines relative to the disc due to a distortion of the disc or a mounting error of the actuator of the optical pickup or the like, there arises a coma aberration in converging light on the recording surface of the disc. Since the amount of this coma aberration is proportional to the cube of the NA value, the result particularly with a high NA value, such as in BDs, would be that only a slight characteristic change of the disc may cause occurrence of quite a large coma aberration.
As the method for achieving adjustment of the focus offset amount and the spherical aberration with high accuracy, for example, there is a method, as exemplified by an information recording/reproduction method disclosed in JP 2007-87483 A, which includes learning a difference between a spherical aberration amount resulting upon a best jitter amount and a spherical aberration amount resulting upon a maximum tracking error signal amplitude, and determining an optimum value of the difference.
However, the conventional information recording/reproduction method disclosed in JP 2007-87483 A has the following problems.
That is, when a spherical aberration amount resulting upon a minimum jitter amount is determined by varying the spherical aberration amount in the process of detecting the minimum jitter amount (servo parameter), there may occur changes in the minimum value of the jitter amount because the jitter is not stabilized in an optical pickup whose servo operation is unstable. Moreover, due to configurational constraints involved in size or thickness reduction of the optical pickup, there may arise cases where it becomes difficult to sufficiently vary the servo parameter such as a spherical aberration correction amount to draw an approximating curve.